1. Field of the Invention
The present invention relates to austenite steels and austenite steel castings using same, and particularly to high-strength heat-resistant austenite steels used for constituent members of thermal power plants or other applications.
2. Background Art
There have been efforts to increase steam temperature for improved efficiency of coal-fired power plants. At present, the highest steam temperature is achieved in a class of coal-fired power plants using USC (Ultra Super Critical) steam turbines, which are operated at a steam temperature of around 620° C. However, this temperature is expected to increase to reduce CO2 emissions. To date, 9Cr and 12Cr heat-resistant ferrite steels have been commonly used for high-temperature members of steam turbines. However, it is believed that use of these steels will be difficult in the future as the steam temperature continues to increase.
A Ni base alloy having a higher working temperature than ferritic steels is a possible candidate alloy of high-temperature members. Ni base alloys contain Al and Ti as alloying elements, and show desirable high-temperature strength because the strengthened phase is the γ′ phase that is stable at high temperatures. As for forging materials, γ′-phase precipitation strengthened alloy is melted as a material ingot using a melting method that involves sophisticated atmosphere control, such as VIM (Vacuum-Induction Melting), ESR (Electroslag Remelting), and VAR (Vacuum-Arc Remelting), and hot forged to produce a product material. In these melting methods, oxidation of active elements Al and Ti during a melting process is prevented by performing the process in a vacuum or by using a slug. In turbine casings and valve casings, the material is typically cast into a shape that relatively resembles the product using a sand mold, and used as a cast material as it is cast. In the casting method, however, melting involves an insufficient barrier against air, and the active elements (Al and Ti) become oxidized when these elements are contained in large amounts.
U.S. Pat. Nos. 3,046,108 and 3,160,500, for example, describe Alloy 625 as an alloy that is applicable to cast materials. This alloy is a solid solution hardening alloy involving a solid solution of Mo and Nb, and can be used as a desirable casting material to also produce thick members without causing defects. It has been confirmed that this alloy has a significantly higher creep capability temperature than common ferritic steels.
JP-A-2012-46796 and JP-A-2011-195880 propose non-γ′ phase precipitation strengthened austenite steels. These are austenite steels that are hardened by precipitation strengthening using intermetallic compounds containing Nb as an alloying element, and show high-temperature strength as Ni3Nb and Fe2Nb precipitate in the grains and in grain boundaries. These materials are produced by melting the material ingot, and used as boiler materials after being processed (hot working).
JP-A-61-147836 proposes a corrosion-resistant austenite steel. This steel is described as having desirable high-temperature strength.
In the production of castings such as turbine casings and valve casings, a molten metal is poured into a mold using a technique such as AOD (Argon Oxygen Decarburization). However, melting of an alloy containing active elements such as Al and Ti, specifically a γ′-phase precipitation strengthened alloy, using this method may result in insufficient high-temperature strength as a result of oxidation of these active elements, which produces Al and Ti contents different from the predetermined contents, or produces oxides that interfere with the process.
The Alloy 625 of U.S. Pat. Nos. 3,046,108 and 3,160,500 is desirable in terms of productivity; however, the proof strength is insufficient, and deformation or loss may occur in a bolted screw when used for, for example, casings. Another drawback is that, when designing a high-strength alloy using a solid solution hardening alloy as a base alloy, the alloy requires further addition of solid solution hardening elements (for example, Mo and Nb). This may result in poor phase stability, causing precipitation of a harmful phase, and problems in long-term phase stability (mechanical characteristics).
The precipitation strengthened alloys of JP-A-2012-46796, JP-A-2011-195880, and JP-A-61-147836 require processes such as forging after the casting process, and are not easily applicable to castings, for example, such as casings.
The γ′-phase precipitation strengthened alloys having high-temperature strength cannot be easily used for castings (particularly, large castings), as described above. The low proof strength of the solid solution hardening alloys is also an issue. In casting production, castability also needs to be considered because macro defects, when occurred frequently during the casting, lead to poor product reliability.